LONDON, March 2 (Reuters Life!) - People who use cannabis in
their youth dramatically increase their risk of psychotic
symptoms, and continued use of the drug can raise the risk of
developing a psychotic disorder in later life, scientists said
on Wednesday.

In a 10-year study of links between cannabis use and
psychosis, Dutch researchers found that cannabis use almost
doubled the risk of later psychotic symptoms.

Experts commenting on the results said the major challenge
for health authorities was to deter enough young people from
using cannabis so that rates of psychosis could be reduced.

"This study adds a further brick to the wall of evidence
showing that use of traditional cannabis is a contributory cause
of psychoses like schizophrenia," said Robin Murray of the
Institute of Psychiatry at Kings College London, who was not
involved in the research.

Wednesday's findings, published in the British Medical
Journal, echo research last year which found that young people
who smoke cannabis for six years or more are twice as likely to
have psychotic episodes, hallucinations or delusions.
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Cannabis is the most commonly used illicit drug in the
world, particularly among adolescents, and is increasingly
linked to added risks of developing mental illness.

But scientists say it is not yet clear whether the link
between cannabis and psychosis is causal, or whether it is
because people with psychosis use cannabis to self-medicate to
calm their symptoms.

For this study, a team of Dutch researchers led by Jim van
Os from Maastricht University studied a random sample of 1,923
adolescents and young adults aged 14 to 24 years.

The study took place in Germany and the researchers
separated out anyone who said they were already using cannabis
and excluded those with pre-existing psychotic symptoms so they
could look at links between new cannabis use and psychosis.

They found that so-called "incident", or new, cannabis use
almost doubled the risk of new psychotic symptoms, even after
accounting for factors such as age, sex, socio-economic status,
use of other drugs and other psychiatric problems.

They also found that in those who were already using
cannabis at the start of the study, continued use increased the
risk of persistent psychotic symptoms. There was no evidence for
self-medication effects since psychotic symptoms did not predict
later cannabis use, they said.

Peter Kinderman, a professor of clinical psychology at the
University of Liverpool, said the study suggested authorities
should take "a cautious and thoughtful approach to cannabis
legislation."

"It's important to remember that psychosis is a very complex
bio-psycho-social phenomenon...but this important paper
certainly reminds us that there's a strong link to the use of
cannabis," he said in an emailed comment.
(Editing by Mark Trevelyan)